Personal trainer Bryant Johnson watches his tiny client pump her body up and down on a green yoga mat, spotting her with his hands at her waist in case she falls. As “PBS NewsHour” blares in the background, Johnson counts down the first of two sets of 10 push-ups.

“Way to go, Justice,” cheers the man building the biceps of Ruth Bader Ginsburg.

By day, Johnson is a records manager in the clerk’s office of the District’s federal court, curating obscure files and trying to avoid paper cuts. In the evening, he drives up the hill — Capitol Hill — to do squats with Ginsburg or take punches from her colleague, Justice Elena Kagan, in the Supreme Court’s ground-floor gym.

The judges, clerks and U.S. marshals who lift and stretch with Johnson may know the law; Johnson, 48, who spent years jumping out of airplanes for the military, knows fitness. But it could be any of a range of professions serving this city’s elite: Johnson is part of the army of Washingtonians in everyday jobs whose lives cross paths with extraordinary power. In his case, that intersection occurs with two of the most influential legal minds in the world.

“Exercise is the great equalizer. It doesn’t matter what size, shape or color you are,” Johnson said in his office cubicle at the federal courthouse on Constitution Avenue, about a half-mile down the hill from the Supreme Court. “A push-up is a push-up, no matter how you look at it.”

Ginsburg and Johnson are an unlikely pair, the world-class lawyer and her physical powerhouse of a trainer. He stands an inch shy of 6 feet, weighs 206 pounds and can pump out 84 push-ups in two minutes. She’s just over 5 feet and just over 100 pounds — and she has passed her own milestone on the green mat.

“When I started, I looked like a survivor of Auschwitz,” Ginsburg said in an interview. “Now I’m up to 20 push-ups.”

And those are old-fashioned, knees-off-the-ground push-ups, her trainer proudly points out.

Discretion is a big part of the unwritten job description for people like Johnson, people who cut hair, cook meals, tailor suits — and keep secrets for those in power. Johnson often knows when his well-known clients are tired or sick — or why they’ve had a rough day on the bench.

In the course of his relationship with Ginsburg, she has written and voted on such major issues of the day as President Obama’s health-care law, a ban on late-term abortion and gender-based employment discrimination.

But Johnson is reluctant to talk about her or betray what she says in the sweaty confines of the gym.

The necessity of exercise, especially for people who sit all day, say, in black robes, created a business opportunity for Johnson that sprang from his day job at the courthouse. But it is Johnson’s style, combining the strictness and professionalism of the military with the sensitivity of a therapist, that has allowed him to gain the trust of more than a half-dozen federal judges and two Supreme Court justices.

Ginsburg began using a personal trainer in 1999, after she was treated for colon cancer and her husband, Martin, who died in 2010, insisted that she hire someone to help her regain her strength. By the justice’s account, she was in bad shape after surgery and radiation.

“I never thought I’d be able to do any of this,” said Ginsburg, who turned 80 on Friday and has survived a second bout with cancer since she began training all those years ago, this time in her pancreas. “I attribute my well-being to our meetings twice a week. It’s essential.”

The private lives of the nine unelected interpreters of the Constitution — even the smallest glimpses of their daily routines in the towering marble building on First Street NE — have long been objects of fascination for the public. Longtime justice Byron White, a football legend whom Ginsburg succeeded in 1993, and Justice Clarence Thomas have shot hoops on a top-floor basketball court known as the “highest court in the land.” Former justice Sandra Day O’Connor still occasionally attends low-impact aerobics and yoga classes at the court. And Kagan has lost weight since she joined the bench in 2010, through a combination of boxing sessions with Johnson and diet.

Johnson drives to the Supreme Court in his white Hummer, which he parks underneath the courthouse after flashing the security badge he was issued with Ginsburg’s name and his picture on it. He punches an access code to enter a ground-floor marble hallway that leads to the gym. After their workouts, his tradition is to escort Ginsburg back to her chambers before leaving the building.

Johnson meets Ginsburg in the justices-only gym (there is another for clerks and other employees). Both of them wear sweats and sneakers. Their hour-long sessions start slowly with a warm-up on the elliptical machine. They move through stretching and weight training and balancing exercises with a rubber fitness ball.

When it’s time for push-ups, Johnson stands guard over Ginsburg, bending down with hands poised to catch her in case her arms give out. “Think of the paperwork I’d have to fill out if something happened to you,” he likes to say.

Johnson was a little nervous about his sessions at the Supreme Court. But the easy rapport he’s developed, particularly with strong female judges, is perhaps natural for someone who was raised largely by his mother, a deaf grandmother and many aunts — women, he says, “who don’t take no mess.”

Neither does Johnson. Growing up on his grandparents’ farm on the Northern Neck of Virginia, Johnson was a hurdler and played cornerback on the football team. But it was his military training that inspired his interest in fitness. An Army reservist attached to the Special Forces, Johnson has traveled the world, jumping out of airplanes, helicopters and a hot-air balloon. Today, his short-cropped hair, broad shoulders and the silver paratrooper wings around his neck are reminders of his military training. But Johnson’s no-nonsense approach in the gym is balanced with an empathetic side.

“He’s a sergeant, but he doesn’t act like a sergeant,” said U.S. District Judge Thomas F. Hogan, whose sessions with Johnson helped him shed some of the pounds he’d put on over the years sitting on the bench.

“You are with this person when they are most vulnerable,” Johnson said. “They show you their weakness, but the point is not to make them feel weak. Your job is to build them up.”

It was one of Johnson’s workout buddies at the courthouse who suggested that he turn his passion for fitness into a business. He began with a colleague from the clerk’s office, but word quickly spread through the courthouse. His client list over the years reads like a who’s who of the federal bench in Washington: Hogan, as well as U.S. District Judges Ellen S. Huvelle, Emmet G. Sullivan and Gladys Kessler.

It was Kessler who sent Ginsburg to Johnson. Ginsburg, in turn, referred Kagan to him.

In the hallways, colleagues often stop Johnson to ask for nutrition tips or compare workout notes. A judge jokingly flexes her arm muscle as she walks by to let him know she’s been working out. He avoids the cafeteria because people have been known to hide or apologize for their unhealthful snacks when they see him coming.

Johnson doesn’t advertise, but he doesn’t have to in the city’s close-knit legal community. As a result of his success, he can charge as much as $120 per hour.

The only interruption in Johnson’s long-standing workouts with Ginsburg came during a three-year deployment to Kuwait. He returned in 2003, and they’ve been back at the leg lifts and chest presses ever since.

Johnson is fond of saying that when a person crosses the threshold of the gym, it doesn’t matter who they are. But when he’s prodding his judicial clients for one last leg curl, he’s sure to add, “Justice, Judge or Your Honor.”

Ann covers legal affairs in the District and Maryland for the Washington Post. Ann previously covered state government and politics in California, New Hampshire and Maryland. She joined the Post in 2005.

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